Sponge grounds are known as nutrient sources, providing nitrate and ammonium to the ocean. We found that they also can do the opposite: in six species from Arctic and North Atlantic sponge grounds, we measured high rates of denitrification, which remove these nutrients from the sea. Rates were highest when the sponge tissue got low in oxygen, which happens when sponges stop pumping because of stress. Sponge grounds may become nutrient sinks when exposed to stress.